So ź is actually more like ś when pronounced (in this context)? At least that's how it sounds to me.... I have never had a language lab course, so I don't know much about all those changes when people speak (that's why I have to ask). Of course I observe people and the way they speak and imitate etc.
I'm not an expert on phonology, however this indeed seems to be a case of consonant devoicing, where an unvoiced consonant (ć) reduces the preceding voiced consonant (ź) to its unvoiced counterpart (ś).